schugh

i currently have a spreadsheet that has a column with only the characters ? [ and E in it, these characters are changed to reflect the availabilty of work, is it possible to have a cell change colour for a period of time (approx 30 mins) when one of the characters is changed to another...e.g if a cell changes from ? to a [ i would like it to fill in red. i am pretty new to the whole excel formatting and would appreciate any assitance on this matter!! thank you in advance